[AD] Expose events in OS X

[Evert Glebbeek <eglebbk@xxxxxxxxxx>]

I don't know whether anyone else has looked into this or plans to do  
it in the future, or even if the following bit of information was  
already discussed, but I looked (briefly) into making it work today  
and found the following:

when a window is exposed, the delegate gets a windowDidExpose message,  
except if the window content is retained (or buffered), in which case  
the message is never generated.
Allegro's windows are buffered. About non-retained windows, Apple's  
documentation says the following:

"The window draws directly to the screen without using any buffer.

You should not use this mode. It exists primarily for use in the  
original Classic Blue Box. It does not support Quartz drawing, alpha  
blending, or opacity. Moreover, it does not support hardware  
acceleration, and interferes with system-wide display acceleration. If  
you use this mode, your application must manage visibility region  
clipping itself, and manage repainting on visibility changes."

I don't think we ever want any of these side effects. That means we  
can never get expose messages on OS X and applications requesting  
expose messages cannot expect them to be actually generated.
This probably needs to be made clear in the documentation, or  
al_create_display() should fail if expose events are required.

Thoughts?

Evert